"Нині неділя." is correct as well and should be accepted.
It had been reported.
The phrase "it is" or"this is" in the English translation would imply the use of the word "це" in the originating Ukrainian sentence. The correct translation should only be "Today is Sunday".
Should Duolingo accept "Сьогодні це неділя"?
I wrote: Sunday is today. It said I was wrong.